Remove never used activity declarations

app/src/main/AndroidManifest.xml

@@ -56,10 +56,6 @@
                 <data android:host="*"/>
             </intent-filter>
         </activity>
-        <activity
-            android:name=".MyPdfThumbActivity"
-            android:exported="true"
-            android:theme="@style/ReaderActivityNoActionBarNoTitle" />
     </application>
 
 </manifest>